The goal of this project is to determine the intracellular distribution of diffusible and structural components within axons, dendrites, glia, and synapses. This is important because the movement of cellular constituents between specific locations defines the dynamic structure, the integrity and the stability of neuronal contacts, and thereby plays a major role in synaptic transmission. Quantitative x-ray microanalysis of the intracellular distribution of chemical elements in parallel fiber/Purkinje cell cerebellar synapses has shown that the calcium content of the smooth endoplasmic reticulum within Purkinje cell dendritic spines increases threefold as a function of synaptic activity. The amount of calcium accumulated in the endoplasmic reticulum is equivalent to 1200 ions per millisecond -- a current that is consistent with calcium entry exclusively through membrane ion channels. Immunocytochemical and in situ hybridization studies of actively myelinating peripheral and central nervous system have demonstrated the location of several myelin proteins and their corresponding mRNAs. The results imply at least three distinct mechanisms of translation and intracellular transport of proteins destined for the myelin sheath, and has led to the identification of a putative transport organelle for the oligodendrocyte cell adhesion molecule, myelin-associated glycoprotein (MAG); this organelle is not present in the Schwann cell where MAG is a distinct molecular species. These results, together with new data on the association between MAG and cytoskeletal proteins, are beginning to define the mechanisms of synthesis, transport, assembly and stabilization of the myelin sheath.